LOS ANGELES, California (CNN) -- In his day, Howard Hughes was as big a heartthrob as Leonardo DiCaprio, who stars as Hughes in the Oscar-nominated "The Aviator." Hollywood folklore links Hughes with an amazing array of leading ladies, including Bette Davis, Gene Tierney, Rita Hayworth and Ginger Rogers.

But there was one actress who truly captured his heart: Katharine Hepburn, played by Cate Blanchett in the film.

Berg took a break from researching his next biography -- of Woodrow Wilson, due in 2009 -- and sat down with CNN to discuss Hepburn's memories of the legendary aviator.

CNN: Can you give us a feel for Hepburn's mood and emotions when she told you about her relationship with Howard Hughes?

A. SCOTT BERG: There was just a wonderful feeling in the room. She obviously felt great tenderness for him. The relationship had been extremely adventurous, but looking back on it, 40, 50 years later, I always felt that she felt it was tinged with some sadness -- I think with what Hughes became, anyways. ...

Whenever she would talk about Hughes, there was always this kind of glint in her eye that suggested that -- of all the relationships in her life -- it might have been the lustiest, in fact. And the stories really ranged from how and when they first met and up until really the last time they encountered each other face to face.

CNN: What do you think drew them together?

BERG: She met Howard Hughes a few years before [Spencer Tracy] and I think that this was at a time when she really didn't want to settle down yet, and here she met this guy who was as independent as she was. Somebody who had come to Hollywood, was going to take it by storm on his own terms -- which was exactly how she came out to Hollywood.

Also, she made very clear to me over and over again, these were two young people who were really infatuated with fame, and each of them wanted to be famous. ... They were in love with each other, but I think as she said they were more in love with the idea of each other and I think each were a very good credit for the other.

CNN: Was there a lot of drama in the relationship?

BERG: My sense is that it it wasn't wildly dramatic as indeed Hepburn's relationship with Spencer Tracy often was, largely because of his alcoholism.

That being said, Howard Hughes was a somewhat explosive personality. He was compulsive not just in his day to day habits but certainly in his womanizing. ...

I think what drew Hepburn, though, was something more positive, which was just this great sense of just being up for anything. This was a guy who just loved stunts, loved thrills, loved his airplane, she learned how to fly because of him -- with him. ... And I think because they were together they could turn it into a game, as opposed to being alone and having to face it -- it would have been a burden.

CNN: What do you think eventually split them apart?

BERG: Hepburn hit a slump in her career when she was labeled box office poison. This wasn't just a rumor around Hollywood; there were posters plastered around town by the independent theater owners saying, "Katharine Hepburn is box office poison." ... I think in that moment, Hepburn again being ever independent, felt more and more that she had to prove herself on her own.

Ironically, this was the exact moment Hughes wanted to get closer and closer, and in fact desperately wanted to marry her. Hughes told a mutual friend of his and mine years later that the biggest mistake he ever made in his life was not being able to convince Katharine Hepburn to marry him. I think the more he wanted to get married, the more she wanted to get away. ...

I think she felt she had to prove it to herself, and above all I think she felt she had to concentrate entirely on herself and her career, and as she said that's a terrible way to enter into a marriage or to exist in a relationship.

BERG: There was one important postscript ... and that is when Hepburn left Hollywood to try to get her career back on track. She decided the way to do that was to come back on Broadway, which she did with a play called "The Philadelphia Story." She very shrewdly bought the film rights for herself. A lot of the advice to do that came from Howard Hughes, and in fact as a kind of parting gesture he helped pay for some of those rights.

CNN: It's kind of ironic. You said before she wanted to stand on her own two feet, make her comeback on her own, and yet Hughes helped her out.

BERG: Yeah, she was not above accepting help, but it had to be within certain margins. And it's that she didn't want anyone to buy her career back. She could have done this on her own, and I think she felt that this was a good business deal, and I think it was also a way of keeping connected to Hughes, because she really did care about him.

CNN: Did her family and friends criticize her for being with Hughes, either while she was with him, or later on when stories about his obsessive-compulsive disorder came out?

BERG: You know, most of us who are under a certain age -- and by that I mean 75 -- really only know Howard Hughes as this kind of weird fellow with long fingernails who was collecting jars of his own urine. But in fact as Hepburn always said this was an absolutely brilliant man. ...

By the time Katharine Hepburn came out to Hollywood in 1932, Howard Hughes was a famous multimillionaire having inherited a lot of money. He had become an extremely successful film producer with three big hits in a row. ... So by the time Katherine Hepburn came out here he was not really a problem to people who knew him, but a rather glamorous figure, and she knew that. And she liked glamour.

CNN: As his condition deteriorated over the years, did she ever feel a sense of regret, a sense of "I could have saved him?"

BERG: There's no doubt that there was kind of a missionary streak to Katherine Hepburn. She loved to save souls, to save lives, to pull things out of the fire. And I think she did feel that she probably could have made some difference in Howard Hughes' later life.

All that being said, I think she felt, he was kind of on a hopeless track. When she knew him, and knew him well, his condition wasn't so exaggerated as it became later on. So by the time she left him he was just exhibiting odd behavior. It really hadn't gone to what she called "the land of coo-coo." ... She had a few theories about that, but I don't think she ever felt that she could quote "cure" him, though.